The present invention relates to a method for operating a nuclear reactor to increase nuclear reactor power and especially a method for increasing nuclear reactor power without danger of failure of the fuel rods disposed in a core of a nuclear reactor.
A core of a nuclear reactor is provided with a plurality of fuel rods where each fuel rod is assembled such that a plurality of fuel pellets are packed in a clad tube. A rapid increase of nuclear reactor power causes pellet-clad-mechanical-interaction in a recognized linear heat generating rate range (indicated as PCI in the following description) between the fuel pellets and the clad tube due to volume expansion of the fuel pellets and causing excessive strain in the clad tube. Thus, there is a possibility of a failure of the fuel rod, which means that one or more than one openings, cracks or pin holes appear on the clad tube so that nuclear fission products are relieved into a surrounding coolant from the fuel rod through the openings, cracks or pin holes. Regarding the failure of the fuel rod, it has been proposed, as described in Japanese Published Patent Application No. 50-143,999, and U.S. Pat. No. 4,057,466 to Thompson et al, to restrain the rate of increase of the linear heat generating rate when the linear heat generating rate is increased above the linear heat generating rate at which the PCI begins.
As disclosed in the Thompson et al patent, in order to properly condition the fuel rods so as to avoid PCI failure, the rate of increase of the fuel power or linear heat generating rate within the pellet-cladding interaction range is maintained below a critical rate of increase. The critical rate of increase necessarily requires a long term of at least several days to attain the desired operating power level for the nuclear reactor and the length of time to attain such level cannot be shortened due to the limitation on the rate of increase.